My green carpet reacts much faster to food than that one. I don't deliberately feed it anymore, it gets enough food fed to it by the clowns and it catches lots of food randomly at feeding time. It is probably 18"+ and hosts my Maroon clown pair.
I've had mine for...gee...maybe 15 years now? My only advice is not to feed it anything too large. I once had mine get really ill. I saved it by reaching into the gut with tweezers and pulling out a large, rotting mass of food. Basically, the food has to be digested before it rots. If it is too large, the 'nem can't digest it fast enough.
Mine has killed a few hermit crabs, snails and one starfish. (I didn't personally witness the starfish...) Mine has never killed a fish, although I'm pretty sure it cleaned up its share of bodies.
Here's a recent photo with the 'nem visible.

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400 gal reef. Established April, 2007. 3 Sequence Dart, RM12-4 skimmer, 2 x OM4Ways, Yellow Tang, Maroon Clown (pair), Blonde Naso Tang, Vlamingi Tang, Foxface Rabbit, Unicorn Tang, 2 Pakistani Butterflies and a few coral gobies
My Tank:
http://www.canreef.com/vbulletin/showthread.php?t=28436